Push-pull moving coil loudspeaker having electromagnetic centering means

ABSTRACT

An electrodynamic acoustic transducer such as a loudspeaker or microphone in which the datum or center position of the diaphragm is determined by two coils. The coils are fixed to one another and to the diaphragm and cut magnetic fields. Datum currents are provided for the coils and the coils are of non-uniform density of winding and/or the magnetic fields are non-uniform. The coils and the datum currents are arranged to provide balanced forces on the coils only when the diaphragm is in the datum position.

United States Patent Manger I 1 51 3,686,446 1451 Aug. 22, 1972 [54] PUSH-PULL MOVING COIL LOUDSPEAKER HAVING ELECTROMAGNETIC CENTERING MEANS [72] Inventor: Josef W. Manger, Karlstadler, 8725 Arnstein, Germany [22] Filed: Dec. 16, 1969 [21] Appl. No.: 885,658

[30] Foreign Application Priority Data Dec. 19, 1968 Germany ..Pl8 15 694.0

52 us. (:1.....179/115.5 0v, 179/1155 vc,

151 1m. (:1. ..H04r 9/02 [58] Field of Search...l79/l 15.5 R, 115.5 PS, 115.5

vc,179/115.5 DV,180

[56] References Cited UNITED STATES PATENTS 1,895,071 1/1933 Fanger ..179/1 15.5 R 2,205,365 6/1940 Schwaen ..179/1 15.5 R 3,163,723 12/1964 Tibbetts ..179/114 A 3,222,463 12/1965 Gorike ..179/181 F 3,196,211 7/1965 Kessenich ..179/115..5 R

2,727,949 12/1955 Lokkesmoe.'.....179/1 15.5 vc 2,926,221 .2/1960 Kagdis ..179/115.5 DV 3,055,991 9/1962 Guss ..179/115.5 v c FOREIGN PATENTS OR APPLICATIONS 312,950 6/1929 Great Britain ..179/1 15.5 R 37,272 l1/l930 France ..179/115.5 VC

OTHER PUBLICATIONS Symmetrical Properties of Transistors and Their Ap-- plications, Sziklai, Proceeding lnst. Radio Engr., June 1953, pp. 717, 719. v

Primary Examiner-Kathleen H. Claffy Assistant ExaminerThomas L. Kundert Attorney--lrving M. Weiner [5 7] ABSTRACT 16 Claims, 7 Drawing Figures PATENTEDwtzwn SHEET 1 BF 4 MVP/110i? JOSEF' w. HANGER Nfi? ATTO RNEY PATENTEDAUBZZ m2 3.ees;44s sum 2 0r 4 Fly. 3 I A2 In ventor JOSEF w. HANGER BY/B47 ATTORNEY PATENTEDMIB Z I972 3.686.446

sum u or 4 lmpli/ua'e I F137. 7

linen/0r.- JOSEF' w. MANGER sY/wy 77. WW

ATTORNEY PUSH-PULL MOVING COIL LOUDSPEAKER HAVING ELECTROMAGNETIC CENTERING MEANS The present invention relates to electro-dynamic transducers. The most commonly used loudspeakers and microphones comprise a cylindrical moving coil in the air-gap of a permanent magnet which provides a radial field through the coil. The electric power supplied to the moving coil is converted into acoustic power by a cone diaphragm attached to the coil.

It is known that the moving parts, that is to say, the moving coil and cone diaphragm, form a vibrating system having a pronounced resonance frequency or frequencies, mainly dependent on the mass of the vibrating system and the return force or spring force. Below the resonance frequency, the power radiated depends substantially on the spring force or compliance, and above the resonance frequency it depends substantially on the mass.

A disadvantage of conventional electro-dynamic loudspeaker of this form is that the resonance frequency of the mass-and-spring vibrating system is generally between 100 and 200 c/s. At lower frequencies, the mechanical vibration amplitude diminishes considerably. For this reason, endeavors are made to reduce the resonance frequency of the mechanical vibrating system as much as possible, which however involves much expenditure and, due to the requirement D m (D spring force, m mass) is accompanied by other disadvantages. Itis true that nowadays low frequencies may be generated with sufficient amplitude by using expensive, large loudspeakers, but the amplitude variation of the original sound is strongly distorted by the processes of vibration decay and buildup.

For improving conventional loudspeaker systems, it is known to use two moving coils and two magnet units, one moving coil, which with its magnet unit is located in front of the diaphragm, supplying a feedback voltage. The intention is to control the amplification by means of this voltage and hence achieve linearization of the frequency variation. It is not possible, however, to compensate the spring-induced drop in vibration amplitude in this way.

In another known form of loudspeaker, the moving coil is coated with iron powder to replace the spring return by a permanent magnet return. This system has not proved satisfactory, however, since leakage of the magnetic field at the edges of the permanent magnet results in a considerable reduction in the return forces, so that return at large amplitudes cannot be ensured.

The present invention is based on a solution to the problem of providing an electro-dynamic transducer which, by simple means, can ensure that even at very low frequencies sufficient amplitude is radiated, and that in the entire audio-frequency range for constant rapidity (deflection amplitude x frequency), there is good proportionality between deflection amplitude and control-current amplitude.

This problem is solved according to the invention in In an electromagnetic return or centering of the diaphragm in the inoperative position, considerable advantages are obtained compared with known loudspeaker systems. On account of the absence of a spring and the resulting low inertia of the system, a very much higher amplitude fidelity is obtained, that is to say, the diaphragm vibration is only very slightly retarded relative to the energizing low-frequency oscillation. A further advantage is that, owing to electromagnetic self-damping, excessive vibration no longer occurs. In conventional loudspeaker systems, for example, in

mechanical deflection of the diaphragm, followed by its release, it is impossible to prevent the diaphragm vibrating about the inoperative position. Such disadvantages are prevented in electromagnetic return. Due to the electromagnetic return according to the invention, not only is good proportionality obtained between the amplitude of the energizing low-frequency current and the mechanical vibration amplitude, but also a substantial increase in the vibration amplitude at low frequencies compared with known loudspeaker systems.

Another substantial advantage of the invention is that the known equation for the vibration amplitude (amplitude equal to electric power supplied divided by the product of the mass and square of the angular frequency) is satisfied almost ideally. Down to at least 15 c/s, no drop in amplitude can be detected in the measured curve.

A transducer according to the invention, when used in loudspeakers, is suitable particularly for high-fidelity reproduction of music.

Apart from the above-mentioned known transducer systems, two other transducers having two moving coils are known. For example, French Patent Specification 970,283 shows a transducer having a double cone with amoving coil at each apex, the double cone not being suspended at the center but being guided slidingly in a cylinder, for increasing reproduction fidelity, while in US. Pat. No. 1,992,300 a loudspeaker arrangement is described, whose two moving coils are intended to prevent excessive heating. In both arrangements, however, the usual means, for example springs, are necessary for middle centering. These known arrangements, despite the two moving coils, also have no electromagnetic middle centering, so that the advantages according to the invention cannot be obtained in their case.

For returning the loudspeaker diaphragm to the inoperative position, according to a preferred embodiment example of the invention, a coil and permanent magnet arrangement is used, the direction of the return force being fixed by the direction of a constant direct current flowing through the coils, as well as by inhomogeneity of the magnetic field and/or non-uniform winding density of the coils.

An embodiment in which two mechanically coupled coils having the same axis and arranged in pot magnets, and in which the electromagnetic forces exerted on the coils in the inoperative position are equal in magnitude but opposite in direction, in all other positions of the coils are also opposite in direction but are different in magnitude owing to inhomogeneous field distribution in the pot magnets, affords the advantage that the distribution of the magnetic field, necessarily present in the pot magnets, can be utilized, without the need to vary the width of the air-gap by means of suitably constructed pole pieces or the winding density of the coils, that is to say, the number of turns per unit length.

With the use of one or more pot magnets combined with moving coils, it is possible either to provide, for the low-frequency current a further moving coil, connected to the diaphragm, arranged separate from the centering device and movable in the field of a further pot magnet, or to superimpose on the low-frequency current the direct current effecting return to the inoperative position, such that the forces impressed on the two coils by the low-frequency current result in a vibration of the coils in rhythm with the low frequency.

In the embodiments described, the diaphragm may be secured either directly to the coils or to a hollow body surrounding the coil-permanent magnet arrangements, which hollow body is in turn rigidly connected to the coils, or the diaphragm may form such a hollow body. The free ends of the diaphragms are separated in the direction of movement by narrow tolerances with respect to the surrounding form (for example a housing or baffle), or by sealing protuberances without any spring effect. Preference is particularly given to the use of elliptical or spherical hollow bodies, giving a substantially less pronounced sound-beaming effect compared with conical funnels.

The invention will now also be described by way of example with reference to the accompanying drawings, in which:'

FIGS. 1, 2, 4 and 6 show embodiments of coil and permanent magnet arrangements according to the invention, and

FIGS. 3 and 5 show circuit arrangements for use with the loudspeaker systems shown in FIGS. 1 to 5;

FIG. 7 shows the dependence of the vibration am plitude on the frequency.

In FIG. 1, a loudspeaker arrangement comprises two, symmetrical, moving-coil and permanent-magnet arrangements 1 and 1a. The permanent magnets consist of pot magnets having the same axis, adjoining each other by their base faces or spaced apart so that they both open outwardly. The magnets are secured to centering spindles 2, 2a, rigidly clamped at their free ends. Two moving coils 5,6 are movable in the annular airgaps L and I. between the yokes and cores of the two pot magnets respectively and have a common axis disposed perpendicular to the lines of flux of magnetic fields cutting said coils. The moving coils 5 and 6 are rigidly secured together by a hollow body 7, which is for example elliptical or spherical. The hollow body 7, which surrounds the moving-coil and permanent-magnet arrangements 1,1ais slidable on the centering spindles 2,2a and consequently moves to-and-fro in rhythm with the moving coils. The plain bearings must be substantially frictionless and without spring effect. It is particularly favorable to arrange rubber stockings on the hollow body which, during the movement of the centering spindle, roll in the longitudinal axis of the latter. Secured to the hollow body 7 in any desired manner is the loudspeaker diaphragm, not show, which may have any form, suitable to the individual case. Preferably, the hollow body may itself act as a diaphragm.

In the embodiment shown as example in FIG. 2, two moving coil and permanent magnet arrangements 8,9 are used, are connected by a rod 10 and are so arranged that they have the same axis of symmetry and are open towards the center of symmetry. A diaphragm 11 is slidable on the rod 10. Two moving coils 12, 13 are movable in the annular air-gaps L L of the permanent magnets, and are connected rigidly together by rods 16,17 secured to the diaphragm 11, or by a hollow cylinder or the like. The diaphragm 11 consequently moves to-and-fro with the same rhythm as the moving coils 12,13.

Since, in the embodiments described, the loudspeaker diaphragm is fixed only to the moving coils, and vibrates together with them, the return produced by spring force, as in conventional loudspeakers, is not possible. Contrary to the known transducers, therefore, return of the diaphragm or moving coils to the starting position is produced by electromagnetic forces.

FIG. 3 shows the basic diagram of a circuit arrangement whereby both the mechanical return of the diaphragm and deflection of the diaphragm are produced in rhythm with the low-frequency current. The circuit arrangement may also be used in combination with the embodiment examples shown in FIGS. 1 and 2, the moving-coil connections denoted by E or A being connected to the moving-coil connections also denoted by E and A respectively. According to FIG. 3, the connections E are short-circuited and are connected to the negative terminal of a battery. The connections A and A of the two coils are connected respectively to the collector of a transistor pair 20,21 whose emitters are connected to the positive terminal of the battery. The base electrodes of the transistors 20,21 are connected together by the secondary winding of a transformer 22, supplying the low frequency. The center tap of the secondary coil is connected in A.C. fashion with a capacitor C to the positive terminal of the battery. The inoperative current adjustment is done by adjusting the base biases by means of two resistors R and R In the embodiments shown in FIGS. 1 and 2 the mov ing coils in the inoperative condition lie symmetrically in the air-gaps L L of the pot magnets. The coil length is preferably rather greater than the length of the pole faces of the pot magnets, so that the coil ends of each coil are already located to some extent in the inhomogeneous magnetic field of the pot magnets.

When the loudspeaker arrangement described is operating, the direct current supplied by the battery must flow through the moving coils in such a manner that one moving coil is deflected in one direction (in FIG. 1 for example the right-hand coil to the right) and the other moving coil in the opposite direction (in FIG. 1 for example the left hand coil to the left). In this case, as long as the two moving coils are in the central position, the direct current has no effect, that is to say an increase or decrease in the direct current or switching on or off of the battery does not result in any deflection of the mechanically connected coils. If, however, the two coils are deflected in the direction of their axes by mechanical vibrations or the like, they are returned automatically to the central position again by the action of the direct current. In the embodiment shown as example in FIG. 1, this is produced because the inhomogeneity of the magnetic fields increases more slowly towards the center of symmetry, that is to say, towards the closed end of the pot magnets or their airgap L L than in the opposite direction. In other words, starting from the embodiment example according to FIG. 1 and assuming that the two moving coils 5,6 are deflected to the left, the force exerted on the right-hand moving coil and resulting from the magnetic field and the direct current, decreases, while the force exerted on the left-hand coil increases. The resultant difference of these two forces pulls the pair of coils back to the central position.

It was assumed in the foregoing analysis that direct current alone flowed through the two coils. It may now be furthermore assumed that by means of the circuit arrangement shown in FIG. 3, the low-frequency is supplied in push-pull operation. By push-pull is here understood phase opposition, that is to say, the direct current flowing through one of the moving coils is weakened, while the direct current flowing through the other moving coil is strengthened, or vice versa. Since the same applies to the forces acting on the moving coils, both coils when energized by low frequency are deflected in the same direction, so that the hollow body 7 (FIG. I) or diaphragm 11 (FIG. 2) together with the moving coils are forced to-and-fro in rhythm with the low frequency. Independently of this, the moving coils are returned to the central position again, due to the return forces producing the deflection.

It is obvious that the invention is not limited to the described examples of FIGS. 1 to 3, but that many modifications are possible. Whereas, for example, in the embodiment according to FIG. I, by suitable choice of the magnetic field, winding direction of the moving coils and the direction of the direct current flowing through the moving coils, it is necessary to ensure that the right-hand moving coil is deflected by the direct current to the right, and the left-hand moving coil is deflected to the left, in the example according to FIG. 2

it is necessary by suitable choice of the said parameters to ensure that the right-hand moving coil is deflected by the direct current to the left and the left-hand mov-' ing coil is deflected to the right, since in this embodiment example the pot magnets are open in the opposite direction. It is also possible, however, to provide moving coil and permanent magnet arrangements in which the difference in the return forces occuring in a deflection is not due to the different inhomogeneity of the pot magnets inwardly or outwardly. For example, different inhomogeneity could be obtained or improved by suitable variation in the gap width of the magnets, by nonuniform winding of the moving coils or by suitable construction of the pole faces at the ends. A fonn will always be selected such that the transmission ratios are optimum, that is to say, good proportionality will be obtained between the amplitude of the energizing low frequency current and the amplitude of the mechanical vibration on the one hand and an almost constant rapidity (angular frequency x amplitude) for all frequencies to be transmitted on the other.

According to FIG. 4, it is furthermore possible to use instead of two pot magnets only one pot magnet 23 and to mount in it two moving coils one behind the other, as shown in FIG. 4. The arrows indicate the directions in which the two coils are deflected by the direct current. This embodiment example has the advantage of permitting cheaper and more compact loudspeaker systems but nevertheless with electromagnetic centering.

Finally, it is also possible to separate completely the centering device from the moving coil forv the low frequency. Such an arrangement is shown diagrammatically in FIG. 6 in which the arrows again indicate the directions in which the individual coils are deflected by the direct current. Such an arrangement has the special advantage that any inhomogeneous magnetic fields may be used for the return without having to accept distortions with regard to sound transmission. In the embodiment example according to FIG. 6, on the contrary, the moving coil for the low frequency may be arranged in a completely homogeneous magnetic field.

The circuit arrangement shown in FIG. 5 concerns a simplification of the circuit arrangement shown in FIG. 3. The connections shown at E and A are to be connected correspondingly to the connections E and A of the embodiment examples shown in FIGS. 1, 2, 4 and 6. In this case, as shown in FIG. 5, reversal of polarity is necessary, compared with the circuit according to FIG. 3, since the direct current flows through the coils in series.

The use of an n-p-n transistor 24 and a p-n-p transistor 25 provides the possibility of like-phase control of the circuit by the low-frequency current instead of push-pull. The use of two batteries B and B is necessary for this purpose. The additional advantage in this circuit arrangement is that in the deflection of the moving coils, the voltages induced in them (Lenz law) are in phase opposition in the circuit path 26. Much better matching with the low-frequency source is thereby made possible, since the input impedance of A to E of the coil arrangement, which is normally subject to relatively large fluctuations (up to about 1:20), due to the back-emf induced by the movement of the coils, behaves almost completely linearly in this circuit arrangement.

Finally, FIG. 7 shows the dependence of the vibration amplitude on the frequency. Curve 27 is typical of a conventional electro-dynamic loudspeaker, while curve 28 shows the behavior of a loudspeaker according to the invention. Owing to the absence of the mechanical natural frequency f (curve 27), caused by the mass/spring system, the amplitude in loudspeakers according to the invention is exceptionally large, even at the lowest frequencies. The relationship of ainplitude equal to the electrical energy divided by the product of the mass and the square of the angular frequency (A= (E)/(M a is satisfied almost ideally.

The configuration of the electromechanical transducer according to the invention permits of absolutely new body forms for the diaphragms. As shown by FIGS. 1 and 2, the diaphragm may be set vibrating in many ways, so that the problem of its clamping no longer arises. In particular, it is no longer necessary to use cone diaphragrns, so that the limitations which these impose, in particular with regard to sound-beaming effects, are eliminated.

The transducer according to the invention may be used not only in loudspeaker systems, but also wherever high demands are laid on the quality of the transducer.

What is claimed is:

1. In an electrodynamic acoustic transducer with a vibratory diaphragm, the provision of centering means for returning the diaphragm to a datum position and including a. two moving coils,

b. means for providing magnetic fields cutting said coils,

c. mechanical means connecting said coils to one another and to said diaphragm,

c. said vibratory diaphragm being arranged to vibrate without any mechanical restraining element affixed between said mechanical means and said means for providing magnetic fields cutting said coils, and

(1. means for providing two datum currents in said coils respectively to interact with said fields and produce balanced forces thereon solely at said datum position,

said diaphragm being returned to said datum position solely by electromagnetic forces,

said two moving coils having a common axis disposed perpendicular to lines of flux of said magnetic fields cutting said coils, and

said two moving coils being spaced apart from each other along the direction of said common axis.

2.,A transducer according to claim 1, wherein said fields are inhomogeneous.

3. A transducer according to claim 1, wherein said coils are wound with non-uniform density of winding.

4. A transducer according to claim 1, wherein said magnetic fields are provided by pot magnets.

5. A transducer according to claim 4, wherein said pot magnets open towards one another.

6. An electrodynamic acoustic transducer with a vibratory diaphragm and centering means for returning said diaphragm to a datum position, said centering means including:

two moving coils;

means for providing magnetic fields cutting said coils;

mechanical means connecting said coils to one another and to said diaphragm;

means for providing two datum currents in said coils respectively to interact with said fields and produce balanced forces thereon solely at said datum position;

said coils being coaxial;

said two moving coils having a common axis disposed perpendicular to lines of flux of said magnetic fields cutting said coils;

said two moving coils being spaced apart from each other along the direction of said common axis;

said magnetic fields being provided by pot magnets;

and

wherein said pot magnets open away from one another.

7. A transducer according to claim 6, wherein said forces acting oppositely on said coils act away from one another.

8. An electrodynamic acoustic transducer with a vibratory diaphragm and centering means for returning said diaphragm to a datum position, said centering means including:

two moving coils; I

means for providing magnetic fields cutting said coils;

mechanical means connecting said coils to one another and to said diaphragm; means for providing two datum currents in said coils respectively to interact with said fields and wherein said forces acting oppositely on said coils act towards one another.

9. An electrodynamic acoustic transducer with a vibratory diaphragm and centering means for returning said diaphragm to a datum position, said centering means including:

two moving coils;

means for providing magnetic fields cutting said coils;

mechanical means connecting said coils to one another and to said diaphragm;

said diaphragm being returned to said datum position solely by electromagnetic forces;

said two moving coils having a common axis disposed perpendicular to lines of flux of said magnetic fields cutting said coils; said two moving coils being spaced apart from each other along the direction of said common axis;

means for providing two datum currents in said coils respectively to interact with said fields and produce balanced forces thereon solely at said datum position; and

wherein said coils are coaxial and said fields are provided by a single pot magnet.

10. A transducer according to claim 1, wherein said coils of the centering means are also operating coils of the transducer and said datum currents are superimposed upon operating currents and produce out-ofbalance forces on said coils.

l l. A transducer according to claim 1, wherein said coils, said magnetic-field providing means, and said mechanical means are surrounded by a hollow body rigidly connected to said coils.

12. A transducer according to claim 11, wherein said hollow body carries said diaphragm.

13. A transducer according to claim 11, wherein said diaphragm is mounted upon said hollow body.

14. An electrodynamic acoustic transducer with a vibratory diaphragm and centering means for returning said diaphragm to a datum position, said centering means including:

two coils;

means for providing magnetic fields cutting said coils; I

mechanical means connecting said coils to one another and to said diaphragm;

means to provide two datum currents in said coils respectively to interact with said fields and produce balanced forces thereon solely at said datum position;

wherein said means to provide said datum currents comprise a circuit arrangement with two batteries to supply said two datum currents respectively in 1 the same sense, and

means for providing two datum currents in said coils respectively to interact with said fields and produce balanced forces thereon solely at said datum position;

said coils, said magnetic-field providing means, and said mechanical means are surrounded by a hollow body rigidly connected to said coils; and

wherein said magnetic fields are provided by potmagnet means and means for coaxial centering are provided in the form of rubber stockings fixed to the hollow body and during axial movement of the hollow body rolling in the axial direction with sliding seating in the hollow body. 

1. In an electrodynamic acoustic transducer with a vibratory diaphragm, the provision of centering means for returning the diaphragm to a datum position and including a. two moving coils, b. means for providing magnetic fields cutting said coils, c. mechanical means connecting said coils to one another and to said diaphragm, c''. said vibratory diaphragm being arranged to vibrate without any mechanical restraining element affixed between said mechanical means and said means for providing magnetic fields cutting said coils, and d. means for providing two datum currents in said coils respectively to interact with said fields and produce balanced forces thereon solely at said datum position, said diaphragm being returned to said datum position solely by electromagnetic forces, said two moving coils having a common axis disposed perpendicular to lines of flux of said magnetic fields cutting said coils, and said two moving coils being spaced apart from each other along the direction of said common axis.
 2. A transducer according to claim 1, wherein said fields are inhomogeneous.
 3. A transducer according to claim 1, wherein said coils are wound with non-uniform density of winding.
 4. A transducer according to claim 1, wherein said magnetic fields are provided by pot magnets.
 5. A transducer according to claim 4, wherein said pot magnets open towards one another.
 6. An electrodynamic acoustic transducer with a vibratory diaphragm and centering means for returning said diaphragm to a datum position, said centering means including: two moving coils; means for providing magnetic fields cutting said coils; mechanical means connecting said coils to one another and to said diaphragm; means for providing two datum currents in said coils respectively to interact with said fields and produce balanced forces thereon solely at said datum position; said coils being coaxial; said two moving coils having a common axis disposed perpendicular to lines of flux of said magnetic fields cutting said coils; said two moving coils being spaced apart from each other along the direction of said common axis; said magnetic fields being provided by pot magnets; and wherein said pot magnets open away from one another.
 7. A transducer according to claim 6, wherein said forces acting oppositely on said coils act away from one another.
 8. An electrodynamic acoustic transducer with a vibratory diaphragm and centering means for returning said diaphragm to a datum position, said centering means including: two moving coils; means for providing magnetic fields cutting said coils; mechanical means connecting said coils to one another and to said diaphragm; means for providing two datum currents in said coils respectively to interact with said fields and produce balanced forces thereon solely at said datum position; said coils being coaxial; said diaphragm being returned to said datum position solely by electromagnetic forces; said two moving coils having a common axis disposed perpendicular to lines of flux of said magnetic fields cutting said coils; said two moving coils being spaced apart from each other along the direction of said common axis; said magnetic fields being provided by pot magnets; said pot magnets open towards one another; and wherein said forces acting oppositely on said coils act towards one another.
 9. An electrodynamic acoustic transducer with a vibratory diaphragm and centering means for returning said diaphragm to a datum position, said centering means including: two moving coils; means for providing magnetic fields cutting said coils; mechanical means connecting said coils to one another and to said diaphragm; said diaphragm being returned to said datum position solely by electromagnetic forces; said two moving coils having a common axis disposed perpendicular to lines of flux of said magnetic fields cutting said coils; said two moving coils being spaced apart from each other along the direction of said common axis; means for providing two datum currents in said coils respectively to interact with said fields and produce balanced forces thereon solely at said datum position; and wherein said coils are coaxial and said fields are provided by a single pot magnet.
 10. A transducer according to claim 1, wherein said coils of the centering means are also operating coils of the transducer and said datum currents are superimposed upon operating currents and produce out-of-balance forces on said coils.
 11. A transducer according to claim 1, wherein said coils, said magnetic-field providing means, and said mechanical means are surrounded by a hollow body rigidly connected to said coils.
 12. A transducer according to claim 11, wherein said hollow body carries said diaphragm.
 13. A transducer according to claim 11, wherein said diaphragm is mounted upon said hollow body.
 14. An electrodynamic acoustic transducer with a vibratory diaphragm and centering means for returning said diaphragm to a datum position, said centering means including: two coils; means for providing magnetic fields cutting said coils; mechanical means connecting said coils to one another and to said diaphragm; means to provide two datum currents in said coils respectively to interact with said fields and produce balanced forces thereon solely at said datum position; wherein said means to provide said datum currents comprise a circuit arrangement with two batteries to supply said two datum currents respectively in the same sense, and means are provided to vary said currents differentially.
 15. A transducer according to claim 14, wherein said batteries are connected to said coils through two transistors respectively and means are provided to vary the base currents of said transistors differentially.
 16. An electrodynamic acoustic transducer with a vibratory diaphragm and centering means for returning said diaphragm to a datum position, said centering means including: two coils; means for providing magnetic fields cutting said coils; mechanical meaNs connecting said coils to one another and to said diaphragm; means for providing two datum currents in said coils respectively to interact with said fields and produce balanced forces thereon solely at said datum position; said coils, said magnetic-field providing means, and said mechanical means are surrounded by a hollow body rigidly connected to said coils; and wherein said magnetic fields are provided by pot-magnet means and means for coaxial centering are provided in the form of rubber stockings fixed to the hollow body and during axial movement of the hollow body rolling in the axial direction with sliding seating in the hollow body. 